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        <div class="span12">
          <h4 id="title">Vector tiles rendered in an offscreen canvas</h4>
          <p class="tags">
            <span class="badge-group">
              <a
                href="./index.html?q=worker" class="badge badge-info">worker</a
              ><a
                class="badge badge-info tag-modal-toggle text-white"
                data-toggle="modal"
                data-target="#tag-example-list"
                data-title="worker"
                data-content="
                  &lt;a class=&quot;list-group-item list-group-item-action active&quot; href=&quot;./offscreen-canvas.html&quot;&gt;Vector tiles rendered in an offscreen canvas&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./worker.html&quot;&gt;Worker&lt;/a&gt;"
                tabindex="0"
                >2</a>
            </span>
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              <a
                href="./index.html?q=offscreencanvas" class="badge badge-info">offscreencanvas</a
              ><a
                class="badge badge-info tag-modal-toggle text-white"
                data-toggle="modal"
                data-target="#tag-example-list"
                data-title="offscreencanvas"
                data-content="
                  &lt;a class=&quot;list-group-item list-group-item-action active&quot; href=&quot;./offscreen-canvas.html&quot;&gt;Vector tiles rendered in an offscreen canvas&lt;/a&gt;"
                tabindex="0"
                >1</a>
            </span>
            <span class="badge-group">
              <a
                href="./index.html?q=vector-tiles" class="badge badge-info">vector-tiles</a
              ><a
                class="badge badge-info tag-modal-toggle text-white"
                data-toggle="modal"
                data-target="#tag-example-list"
                data-title="vector-tiles"
                data-content="
                  &lt;a class=&quot;list-group-item list-group-item-action active&quot; href=&quot;./offscreen-canvas.html&quot;&gt;Vector tiles rendered in an offscreen canvas&lt;/a&gt;"
                tabindex="0"
                >1</a>
            </span>
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          <div id="map" class="map">
  <pre id="info" class="info"/>
</div>
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      <div class="row-fluid">
        <div class="span12">
          <p id="shortdesc">Example of a map that delegates rendering to a worker.</p>
          <div id="docs"><p>The map in this example is rendered in a web worker, using <code>OffscreenCanvas</code>. <strong>Note:</strong> This is currently only supported in Chrome and Edge.</p>
</div>
        </div>
      </div>

      <div class="row-fluid">
        <h5 class="source-heading">main.js</h5>
        <pre><code id="example-js-source" class="language-js">import 'ol/ol.css';
import Layer from &#x27;ol/layer/Layer&#x27;;
import Map from &#x27;ol/Map&#x27;;
import Source from &#x27;ol/source/Source&#x27;;
import View from &#x27;ol/View&#x27;;
import stringify from &#x27;json-stringify-safe&#x27;;
import {FullScreen} from &#x27;ol/control&#x27;;
import {
  compose,
  create,
  toString as toTransformString,
} from &#x27;ol/transform&#x27;;
import {createXYZ} from &#x27;ol/tilegrid&#x27;;

const worker &#x3D; new Worker(&#x27;./worker.js&#x27;, {type: &#x27;module&#x27;});

let container,
  transformContainer,
  canvas,
  rendering,
  workerFrameState,
  mainThreadFrameState;

// Transform the container to account for the differnece between the (newer)
// main thread frameState and the (older) worker frameState
function updateContainerTransform() {
  if (workerFrameState) {
    const viewState &#x3D; mainThreadFrameState.viewState;
    const renderedViewState &#x3D; workerFrameState.viewState;
    const center &#x3D; viewState.center;
    const resolution &#x3D; viewState.resolution;
    const rotation &#x3D; viewState.rotation;
    const renderedCenter &#x3D; renderedViewState.center;
    const renderedResolution &#x3D; renderedViewState.resolution;
    const renderedRotation &#x3D; renderedViewState.rotation;
    const transform &#x3D; create();
    // Skip the extra transform for rotated views, because it will not work
    // correctly in that case
    if (!rotation) {
      compose(
        transform,
        (renderedCenter[0] - center[0]) / resolution,
        (center[1] - renderedCenter[1]) / resolution,
        renderedResolution / resolution,
        renderedResolution / resolution,
        rotation - renderedRotation,
        0,
        0
      );
    }
    transformContainer.style.transform &#x3D; toTransformString(transform);
  }
}

const map &#x3D; new Map({
  layers: [
    new Layer({
      render: function (frameState) {
        if (!container) {
          container &#x3D; document.createElement(&#x27;div&#x27;);
          container.style.position &#x3D; &#x27;absolute&#x27;;
          container.style.width &#x3D; &#x27;100%&#x27;;
          container.style.height &#x3D; &#x27;100%&#x27;;
          transformContainer &#x3D; document.createElement(&#x27;div&#x27;);
          transformContainer.style.position &#x3D; &#x27;absolute&#x27;;
          transformContainer.style.width &#x3D; &#x27;100%&#x27;;
          transformContainer.style.height &#x3D; &#x27;100%&#x27;;
          container.appendChild(transformContainer);
          canvas &#x3D; document.createElement(&#x27;canvas&#x27;);
          canvas.style.position &#x3D; &#x27;absolute&#x27;;
          canvas.style.left &#x3D; &#x27;0&#x27;;
          canvas.style.transformOrigin &#x3D; &#x27;top left&#x27;;
          transformContainer.appendChild(canvas);
        }
        mainThreadFrameState &#x3D; frameState;
        updateContainerTransform();
        if (!rendering) {
          rendering &#x3D; true;
          worker.postMessage({
            action: &#x27;render&#x27;,
            frameState: JSON.parse(stringify(frameState)),
          });
        } else {
          frameState.animate &#x3D; true;
        }
        return container;
      },
      source: new Source({
        attributions: [
          &#x27;&lt;a href&#x3D;&quot;https://www.maptiler.com/copyright/&quot; target&#x3D;&quot;_blank&quot;&gt;© MapTiler&lt;/a&gt;&#x27;,
          &#x27;&lt;a href&#x3D;&quot;https://www.openstreetmap.org/copyright&quot; target&#x3D;&quot;_blank&quot;&gt;© OpenStreetMap contributors&lt;/a&gt;&#x27;,
        ],
      }),
    }),
  ],
  target: &#x27;map&#x27;,
  view: new View({
    resolutions: createXYZ({tileSize: 512}).getResolutions(),
    center: [0, 0],
    zoom: 2,
  }),
});
map.addControl(new FullScreen());

map.on(&#x27;pointermove&#x27;, function (evt) {
  if (evt.dragging) {
    return;
  }
  const pixel &#x3D; map.getEventPixel(evt.originalEvent);
  worker.postMessage({
    action: &#x27;requestFeatures&#x27;,
    pixel: pixel,
  });
});

// Worker messaging and actions
worker.addEventListener(&#x27;message&#x27;, (message) &#x3D;&gt; {
  if (message.data.action &#x3D;&#x3D;&#x3D; &#x27;loadImage&#x27;) {
    // Image loader for ol-mapbox-style
    const image &#x3D; new Image();
    image.crossOrigin &#x3D; &#x27;anonymous&#x27;;
    image.addEventListener(&#x27;load&#x27;, function () {
      createImageBitmap(image, 0, 0, image.width, image.height).then(
        (imageBitmap) &#x3D;&gt; {
          worker.postMessage(
            {
              action: &#x27;imageLoaded&#x27;,
              image: imageBitmap,
              src: message.data.src,
            },
            [imageBitmap]
          );
        }
      );
    });
    image.src &#x3D; message.data.src;
  } else if (message.data.action &#x3D;&#x3D;&#x3D; &#x27;getFeatures&#x27;) {
    showInfo(message.data.features);
  } else if (message.data.action &#x3D;&#x3D;&#x3D; &#x27;requestRender&#x27;) {
    // Worker requested a new render frame
    map.render();
  } else if (canvas &amp;&amp; message.data.action &#x3D;&#x3D;&#x3D; &#x27;rendered&#x27;) {
    // Worker provies a new render frame
    requestAnimationFrame(function () {
      const imageData &#x3D; message.data.imageData;
      canvas.width &#x3D; imageData.width;
      canvas.height &#x3D; imageData.height;
      canvas.getContext(&#x27;2d&#x27;).drawImage(imageData, 0, 0);
      canvas.style.transform &#x3D; message.data.transform;
      workerFrameState &#x3D; message.data.frameState;
      updateContainerTransform();
    });
    rendering &#x3D; false;
  }
});

const info &#x3D; document.getElementById(&#x27;info&#x27;);
function showInfo(propertiesFromFeatures) {
  if (propertiesFromFeatures.length &#x3D;&#x3D; 0) {
    info.innerText &#x3D; &#x27;&#x27;;
    info.style.opacity &#x3D; 0;
    return;
  }
  const properties &#x3D; propertiesFromFeatures.map((e) &#x3D;&gt;
    Object.keys(e)
      .filter((key) &#x3D;&gt; !key.includes(&#x27;:&#x27;))
      .reduce((newObj, currKey) &#x3D;&gt; ((newObj[currKey] &#x3D; e[currKey]), newObj), {})
  );
  info.innerText &#x3D; JSON.stringify(properties, null, 2);
  info.style.opacity &#x3D; 1;
}
</code></pre>
      </div>

      <div class="row-fluid">
        <h5 class="source-heading">index.html</h5>
        <pre><code id="example-html-source" class="language-markup">&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;
  &lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;title&gt;Vector tiles rendered in an offscreen canvas&lt;/title&gt;
    &lt;!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer --&gt;
    &lt;script src="https://unpkg.com/elm-pep@1.0.6/dist/elm-pep.js"&gt;&lt;/script&gt;
    &lt;!-- The lines below are only needed for old environments like Internet Explorer and Android 4.x --&gt;
    &lt;script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,TextDecoder"&gt;&lt;/script&gt;
    &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/core-js/3.18.3/minified.js"&gt;&lt;/script&gt;
    &lt;style&gt;
      .map {
        width: 100%;
        height:400px;
      }
      .map {
        background: rgba(232, 230, 223, 1);
        position: relative;
      }
      .map .ol-rotate {
        left: .5em;
        bottom: .5em;
        top: auto;
        right: auto;
      }

      .info {
        z-index: 1;
        opacity: 0;
        position: absolute;
        bottom: 0;
        left: 0;
        margin: 0;
        background: rgba(0,60,136,0.7);
        color: white;
        border: 0;
        transition: opacity 100ms ease-in;
      }
    &lt;/style&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;div id&#x3D;&quot;map&quot; class&#x3D;&quot;map&quot;&gt;
      &lt;pre id&#x3D;&quot;info&quot; class&#x3D;&quot;info&quot;/&gt;
    &lt;/div&gt;    &lt;script src="main.js"&gt;&lt;/script&gt;
  &lt;/body&gt;
&lt;/html&gt;</code></pre>
      </div>

      <div class="row-fluid extra-source">
        <h5 class="source-heading">worker.js</h5>
        <pre><code class="language-js">import MVT from &#x27;ol/format/MVT&#x27;;
import TileQueue, {
  getTilePriority as tilePriorityFunction,
} from &#x27;ol/TileQueue&#x27;;
import VectorTileLayer from &#x27;ol/layer/VectorTile&#x27;;
import VectorTileSource from &#x27;ol/source/VectorTile&#x27;;
import stringify from &#x27;json-stringify-safe&#x27;;
import styleFunction from &#x27;ol-mapbox-style/dist/stylefunction&#x27;;
import {get} from &#x27;ol/proj&#x27;;
import {inView} from &#x27;ol/layer/Layer&#x27;;

/** @type {any} */
const worker &#x3D; self;

let frameState, pixelRatio, rendererTransform;
const canvas &#x3D; new OffscreenCanvas(1, 1);
// OffscreenCanvas does not have a style, so we mock it
canvas.style &#x3D; {};
const context &#x3D; canvas.getContext(&#x27;2d&#x27;);

const sources &#x3D; {
  landcover: new VectorTileSource({
    maxZoom: 9,
    format: new MVT(),
    url: &#x27;https://api.maptiler.com/tiles/landcover/{z}/{x}/{y}.pbf?key&#x3D;Get your own API key at https://www.maptiler.com/cloud/&#x27;,
  }),
  contours: new VectorTileSource({
    minZoom: 9,
    maxZoom: 14,
    format: new MVT(),
    url: &#x27;https://api.maptiler.com/tiles/contours/{z}/{x}/{y}.pbf?key&#x3D;Get your own API key at https://www.maptiler.com/cloud/&#x27;,
  }),
  openmaptiles: new VectorTileSource({
    format: new MVT(),
    maxZoom: 14,
    url: &#x27;https://api.maptiler.com/tiles/v3/{z}/{x}/{y}.pbf?key&#x3D;Get your own API key at https://www.maptiler.com/cloud/&#x27;,
  }),
};
const layers &#x3D; [];

// Font replacement so we do not need to load web fonts in the worker
function getFont(font) {
  return font[0].replace(&#x27;Noto Sans&#x27;, &#x27;serif&#x27;).replace(&#x27;Roboto&#x27;, &#x27;sans-serif&#x27;);
}

function loadStyles() {
  const styleUrl &#x3D;
    &#x27;https://api.maptiler.com/maps/topo/style.json?key&#x3D;Get your own API key at https://www.maptiler.com/cloud/&#x27;;

  fetch(styleUrl)
    .then((data) &#x3D;&gt; data.json())
    .then((styleJson) &#x3D;&gt; {
      const buckets &#x3D; [];
      let currentSource;
      styleJson.layers.forEach((layer) &#x3D;&gt; {
        if (!layer.source) {
          return;
        }
        if (currentSource !&#x3D;&#x3D; layer.source) {
          currentSource &#x3D; layer.source;
          buckets.push({
            source: layer.source,
            layers: [],
          });
        }
        buckets[buckets.length - 1].layers.push(layer.id);
      });

      const spriteUrl &#x3D;
        styleJson.sprite + (pixelRatio &gt; 1 ? &#x27;@2x&#x27; : &#x27;&#x27;) + &#x27;.json&#x27;;
      const spriteImageUrl &#x3D;
        styleJson.sprite + (pixelRatio &gt; 1 ? &#x27;@2x&#x27; : &#x27;&#x27;) + &#x27;.png&#x27;;
      fetch(spriteUrl)
        .then((data) &#x3D;&gt; data.json())
        .then((spriteJson) &#x3D;&gt; {
          buckets.forEach((bucket) &#x3D;&gt; {
            const source &#x3D; sources[bucket.source];
            if (!source) {
              return;
            }
            const layer &#x3D; new VectorTileLayer({
              declutter: true,
              source,
              minZoom: source.getTileGrid().getMinZoom(),
            });
            layer.getRenderer().useContainer &#x3D; function (target, transform) {
              this.containerReused &#x3D; this.getLayer() !&#x3D;&#x3D; layers[0];
              this.canvas &#x3D; canvas;
              this.context &#x3D; context;
              this.container &#x3D; {
                firstElementChild: canvas,
                style: {
                  opacity: layer.getOpacity(),
                },
              };
              rendererTransform &#x3D; transform;
            };
            styleFunction(
              layer,
              styleJson,
              bucket.layers,
              undefined,
              spriteJson,
              spriteImageUrl,
              getFont
            );
            layers.push(layer);
          });
          worker.postMessage({action: &#x27;requestRender&#x27;});
        });
    });
}

// Minimal map-like functionality for rendering
const tileQueue &#x3D; new TileQueue(
  (tile, tileSourceKey, tileCenter, tileResolution) &#x3D;&gt;
    tilePriorityFunction(
      frameState,
      tile,
      tileSourceKey,
      tileCenter,
      tileResolution
    ),
  () &#x3D;&gt; worker.postMessage({action: &#x27;requestRender&#x27;})
);

const maxTotalLoading &#x3D; 8;
const maxNewLoads &#x3D; 2;

worker.addEventListener(&#x27;message&#x27;, (event) &#x3D;&gt; {
  if (event.data.action &#x3D;&#x3D;&#x3D; &#x27;requestFeatures&#x27;) {
    const layersInView &#x3D; layers.filter((l) &#x3D;&gt;
      inView(l.getLayerState(), frameState.viewState)
    );
    const observables &#x3D; layersInView.map((l) &#x3D;&gt;
      l.getFeatures(event.data.pixel)
    );
    Promise.all(observables).then((res) &#x3D;&gt; {
      const features &#x3D; res.flat();
      worker.postMessage({
        action: &#x27;getFeatures&#x27;,
        features: JSON.parse(stringify(features.map((e) &#x3D;&gt; e.getProperties()))),
      });
    });
    return;
  }

  if (event.data.action !&#x3D;&#x3D; &#x27;render&#x27;) {
    return;
  }
  frameState &#x3D; event.data.frameState;
  if (!pixelRatio) {
    pixelRatio &#x3D; frameState.pixelRatio;
    loadStyles();
  }
  frameState.tileQueue &#x3D; tileQueue;
  frameState.viewState.projection &#x3D; get(&#x27;EPSG:3857&#x27;);
  layers.forEach((layer) &#x3D;&gt; {
    if (inView(layer.getLayerState(), frameState.viewState)) {
      const renderer &#x3D; layer.getRenderer();
      renderer.renderFrame(frameState, canvas);
    }
  });
  layers.forEach(
    (layer) &#x3D;&gt; layer.getRenderer().context &amp;&amp; layer.renderDeclutter(frameState)
  );
  if (tileQueue.getTilesLoading() &lt; maxTotalLoading) {
    tileQueue.reprioritize();
    tileQueue.loadMoreTiles(maxTotalLoading, maxNewLoads);
  }
  const imageData &#x3D; canvas.transferToImageBitmap();
  worker.postMessage(
    {
      action: &#x27;rendered&#x27;,
      imageData: imageData,
      transform: rendererTransform,
      frameState: JSON.parse(stringify(frameState)),
    },
    [imageData]
  );
});
</code></pre>
      </div>
      <div class="row-fluid">
        <h5 class="source-heading">package.json</h5>
        <pre><code id="example-pkg-source" class="language-json">{
  &quot;name&quot;: &quot;offscreen-canvas&quot;,
  &quot;dependencies&quot;: {
    &quot;ol&quot;: &quot;6.13.0&quot;,
    &quot;json-stringify-safe&quot;: &quot;^5.0.1&quot;
  },
  &quot;devDependencies&quot;: {
    &quot;parcel&quot;: &quot;^2.0.0&quot;
  },
  &quot;scripts&quot;: {
    &quot;start&quot;: &quot;parcel index.html&quot;,
    &quot;build&quot;: &quot;parcel build --public-url . index.html&quot;
  }
}</code></pre>
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          fetch(link, {method: 'head'}).then(function(response) {
            var a = document.getElementById('latest-link');
            a.href = response.status == 200 ? link : '../../latest/examples/';
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